The experiments described in this proposal are focused on understanding the mechanisms underlying the maintenance of stable heritable states of gene expression during development by the Polycomb Group (PcG) and Trithorax Group (trxG) proteins. PcG and trxG proteins are now implicated in many biological processes, including genome-wide control of transcriptional programs, maintenance of cell fates, stem cell self-renewal, X- inactivation, tissue regeneration, and reprogramming of gene expression patterns at the onset of differentiation. Polycomb silencing mechanisms are also directly implicated in the aberrant silencing of tumor suppressor genes and others in cancer. The underlying mechanisms that regulate Polycomb silencing are little understood. The Drosophila homeotic genes remain a preeminent source of new insights into these mechanisms. The aims of proposed work are 1) to further investigate the regulation of the TRX-dependent acetylation of histone H3K27 by CBP, the key mechanism by which the TRX prevents Polycomb silencing. 2) investigate the collaboration of the histone demethylase UTX in TRX-dependent H3K27 acetylation and its role during different developmental stages, including involvement in reversing Polycomb silencing at the onset of differentiation 3) investigate whether ASH1 also mediates H3K27 acetylation through its known association with CBP. We also plan to investigate the mechanism underlying the heritable transmission of Polycomb silencing during the cell division cycle, which occurs with high fidelity throughout development. Understanding the role of these new factors in regulating Polycomb silencing will provide new insights into the mechanisms underlying the epigenetic inheritance of stable chromatin states during development. PUBLIC HEALTH RELEVANCE: This proposal focuses on understanding the mechanisms underlying the maintenance of stable states of gene expression by the Polycomb and Trithorax complexes. It explores the role of newly identified proteins and enzyme activities associated with Polycomb and Trithorax complexes in regulating transcriptionally active and silent chromatin states. It also explores the basis of the remarkable fidelity with which Polycomb silencing is "epigenetically" transmitted to daughter cells during cell division. Polycomb silencing mechanisms have recently been implicated in stem cell self-renewal and pluripotency. Abnormal over-expression of Polycomb proteins, which occurs in many tumors, also underlies the widespread aberrant silencing of critical tumor suppressor genes. This research will lead to a deeper understanding of the fundamental mechanisms underlying Polycomb silencing and the mechanisms that regulate it. It will provide new insights into the maintenance of cell fates, genome reprogramming for differentiation, and have broad implications for cancer and stem cell biology.